We will study synaptic integration, regional membrane properties, and control of internal calcium in neurons of the supraesophageal ganglion of the giant barnacle Balanus nubilus. We are particularly, but not exclusively, interested in these properties in neurons in the visual pathway where information is processed and transmitted by graded potentials and not action potentials. The barnacle ganglion has several advantageous morphological and physiological features for studying these properties. Most of these events occur on the dendrites or presynaptic terminals of cells and hence cannot be easily examined using microelectrodes. We will employ optical methods using voltage-senstive dyes to record simultaneously from many positions on individual cells, including fine processes. With this technique we will determine passive membrane properties at many positions on the cell, regional variations in ionic conductance mechanisms, synapse locations and the propagation of synaptic potentials within a cell. For some selected neurons, we will attempt to combine all of these properties into a unified electrical model. In addition, we will examine the control of internal calcium in the presynaptic terminal of the median photoreceptor. This is an example of a non-fatiguing, graded synapse. With the same apparatus used for the membrane potential measurements we will measure changes in absorption in the dye Arsenazo III to indicate changes in internal calcium at different positions and under a variety of physiological conditions in the photoreceptor.